This invention relates generally to magnetic resonance imaging (MRI) and, more particularly the invention relates to detecting a long spin echo signal train using spiral k-space coverage.
Nuclear magnetic resonance (NMR) imaging, also called magnetic resonance imaging (MRI), is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is completely non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
Briefly, a strong static magnetic field is employed to line up atoms whose nuclei have an odd number of protons and/or neutrons, that is, have spin angular momentum and a magnetic dipole moment. A second RF magnetic field, applied as a single pulse transverse to the first, is then used to pump energy into these nuclei, flipping them over, for example to 90.degree. or 180.degree.. After excitation the nuclei gradually return to alignment with the static field and give up the energy in the form of weak but detectable free induction decay (FID). These FID signals are used by a computer to produce images.
The excitation frequency, and the FID frequency, is defined by the Larmor relationship which states that the angular frequency .omega..sub.0, of the precession of the nuclei is the product of the magnetic field B.sub.0, and the so-called magnetogyric ratio, .gamma., a fundamental physical constant for each nuclear species: EQU .omega..sub.o B.sub.0 .multidot..gamma.
Accordingly, by superimposing a linear gradient field, B.sub.z =Z.multidot.G.sub.z, on the static uniform field, B.sub.0, which defined Z axis, for example, nuclei in a selected X-Y plane can be excited by proper choice of the frequency spectrum of the transverse excitation field applied along the X or Y axis. Similarly, a gradient field can be applied in the X-Y plane during detection of the FID signals to spatially localize the FID signals in the plane. The angle of nuclei spin flip in response to an RF pulse excitation proportional to the integral of the pulse over time.
Hennig et al., "RARE Imaging: A Fast Imaging Method for Clinical MR," Magnetic Resonance in Medicine 3, 823-833 (1986) discloses an RF pulse sequence for obtaining a pulse echo train for heavily T.sub.2 weighted images in a single shot. Reordering the acquisitions and using multiple interleaved acquisitions produces very high quality images with contrast corresponding to arbitrary echo times. In order to return to single shot acquisition, Oshio et al., "Single Shot GRASE Imaging Without Fast Gradients," Magnetic Resonance in Medicine, 26(2):355-360, August 1992, disclose a GRASE RF pulse sequence which combines a RARE echo train with several gradient recalled echoes per RARE echo. This can be thought of as a hybrid between RARE and echo-planar imaging (EPI) which speeds up RARE, but off-resonance constraints restrict the effective echo times that can be produced.
The present invention is directed to a combination of RARE and Fast Spiral Imaging disclosed in Meyer et al., "Fast Spiral Coronary Artery Imaging," Magnetic Resonance in Medicine, 28(2):202-213, December 1992. This has several advantages since spirals very efficiently cover k-space. On conventional gradient systems the required data acquisition time is reduced compared to echo planar. Part of this reduction comes from not collecting the corners of k-space. The other part comes from not wasting time constantly doubling back as echo-planar does. Another major consideration is the suppression of off-resonance effects.